Nucleic acids and proteins showing increased expression dose under salt stress

ABSTRACT

A nucleic acid and a protein increasing expression levels under salt stress provide a nucleic acid with a nucleic acid sequence of SEQ. ID. NO. 1 in Sequence Listing, a variant nucleic acid thereof, a protein with an amino acid sequence of SEQ. ID. NO. 2 in Sequence Listing, and a variant protein thereof. Each of the nucleic acids and the proteins increases an expression level under salt stress. The nucleic acid and the protein are capable of providing a novel protein with a function to impart salt stress tolerance to a plant, and providing a novel gene encoding the novel protein.

TECHNICAL FIELD

The present invention relates to a novel protein which increases anexpression level under salt stress and to a novel gene encoding theprotein.

BACKGROUND ART

Most of the soil in dry regions or coastal areas on earth contains saltsand therefore is not deemed favorable for vegetation. There are problemsthat crops cannot grow in such lands, or cannot reap sufficient harvestseven if the crops can grow there. However, development of crops whichcan grow in such lands has been eagerly expected to deal with expansionof dry regions attributable to global warming and to deal withpopulation increases in developing countries. Accordingly, there is anurgent need to develop salt-tolerant crops by means of breeding orgenetic engineering.

Meanwhile, most of plants growing on earth are exposed to variousenvironmental stresses such as high temperature, low temperature, dryweather, and high salinity. The plants continue to grow by exertingresistance to those stresses in some way. To be more precise, it isknown that various stress response genes operate when a plant is exposedto the foregoing environmental stresses, and the plant shows resistanceto the environmental stresses by performing physiological responses atthe cellular level. The genes considered to cause the plant to exertsuch a function have been already isolated by a subtractionhybridization method and by a differential screening method.

However, it is considered that there are enormous numbers of genesresponsive to environmental stresses as the plants show variousinductions of gene expressions and inhibition patterns depending ondifferences in environmental stress factors and in plant species.Accordingly, the present situation is still far to isolation of all therelevant genes, and isolation of these genes is now in the process ofenergetic efforts.

Meanwhile, there are two methods to produce or breed a salt stresstolerant plant, namely, a method of producing a salt stress toleranttransgenic plant by artificially introducing a gene related to saltstress tolerance into a plant cell, and a method of introducing the genetaken from a salt stress tolerant plant into a plant targeted forbreeding by use of crossing technologies. In order to realize these, itis necessary to isolate the gene(s) related to salt stress tolerance andclarify functions thereof.

Some genes which are subjected to induction of expression under saltstress have been known to exist to date (Yao, A., Molecular biology ofsalt tolerance in the context of whole-plant physiology, J. Exp. Bot.,49, 915–929 (1998), Nelson, D. E., Shen, B., and Bohnert, H. J. Salinitytolerance-mechanisms, models and the metabolic engineering of complextraits, Genetic Engineering, 20, 153–176 (1998)). However, in order todevelop a plant having higher tolerance, it is considered to beimportant to isolate more genes related to salt stress tolerance toprogress in functional analyses thereof.

DISCLOSURE OF THE INVENTION

The present invention has been made in consideration of theabove-mentioned problem of the prior art. It is an object of the presentinvention to provide a novel protein which increases an expression levelunder salt stress and has a function to equip a plant with salt stresstolerance, and to provide a novel gene encoding the protein.

As a result of extensive researches for attaining the object, theinventors of the present invention have found out a Sub4 gene, which isthe novel gene that increases an expression level under salt stress, andthus have consummated the present invention.

Specifically, a nucleic acid of the present invention to be subjected tothe induction of the expression under salt stress is a nucleic acidincluding a nucleic acid sequence of SEQ. ID. NO. 1 in Sequence Listing.

Moreover, another nucleic acid of the present invention to be subjectedto the induction of the expression under salt stress is a nucleic acidincluding part of a nucleic acid sequence of SEQ. ID. NO. 1 in SequenceListing.

Here, yet another nucleic acid of the present invention to be subjectedto induction of the expression under salt stress may be a nucleic acidwhich hybridizes under a stringent condition with any one of theforegoing nucleic acids or with a nucleic acid having a complementarynucleic acid sequence to any one of the foregoing nucleic acids.

Moreover, still another nucleic acid of the present invention to besubjected to induction of the expression under salt stress is a nucleicacid including a nucleic acid sequence encoding an amino acid sequenceof SEQ. ID. NO. 2 in Sequence Listing.

Meanwhile, a protein of the present invention to be subjected toinduction of the expression under salt stress is a protein including theamino acid sequence of SEQ. ID. NO. 2 in Sequence Listing.

Here, the protein of the present invention to be subjected to inductionof the expression under salt stress may include an amino acid sequence,which has any of substitution, deletion, insertion and addition of atleast one amino acid in the amino acid sequence of SEQ. ID. NO. 2 inSequence Listing, and which increases an expression level under saltstress.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an electrophoretic photograph showing a result of identifyinga gene showing expression specifically in salt stress tolerant barley byuse of the Northern analysis.

FIG. 2 is an electrophoretic photograph of expression patterns detectedby the SDS-PAGE regarding a transcription product of a Sub4 gene showingexpression specifically in the salt stress tolerant barley, in the casewhere the transcription product induce the expression in Escherichiacoli.

FIG. 3 is a graph showing relations between incubation time and growthrates under various conditions of salt concentrations concerning thegenetically transformed Escherichia coli including the Sub4 gene showingexpression specifically in the salt stress tolerant barley.

BEST MODES FOR CARRYING OUT THE INVENTION

Now, a preferred embodiment of the present invention will be describedin detail.

A “nucleic acid” in the present invention means a deoxyribonucleic acid(DNA), a ribonucleic acid (RNA), or a polynucleotide which can be anactive DNA or RNA being subjected to induction. Preferably, the nucleicacid is the DNA and/or the RNA.

Moreover, “to hybridize under a stringent condition” in the presentinvention means mutual hybridization of two nucleic acid fragments undera hybridization condition as described in Sambrook, J. et al,“Expression of cloned genes in E. coli” (Molecular Cloning: A laboratorymanual (1989)) Cold Spring Harbor Laboratory Press, New York, USA,9.47–9.62 and 11.45–11.61.

To be more precise, the “stringent condition” means performinghybridization in 6.0×SSC at about 45° C. and then washing in 2.0×SSC at50° C. In order to select stringency, the salt concentration in thewashing process can be selected in a range from about 2.0×SSC at 50° C.as low stringency to about 0.2×SSC at 50° C. as high stringency.Moreover, the temperature in the washing process can be elevated fromthe room temperature at about 22° C. of the low stringency condition upto about 65° C. of the high stringency condition.

Firstly, description will be made regarding a nucleic acid of thepresent invention.

The nucleic acid of the present invention is found in and isolated froma root of a variety of barley having highly tolerant to salt stress, andis characterized by increasing an expression level or exhibitingexpression specifically under salt stress. The nucleic acid includes anucleic acid sequence of SEQ. ID. NO. 1 in Sequence Listinging, whichhas 1377 bases. Here, the “isolated” state means a nucleic acid orpolypeptide, which does not virtually contain a cell material and aculture medium when the nucleic acid or the polypeptide is formed by theDNA recombination technology or does not virtually contain a precursorchemical substance or other chemical substances when the nucleic acid orthe polypeptide is chemically synthesized.

In this specification, the condition “under the salt stress” means anunfavorable condition for vegetation attributable to an elevation in thesodium chloride concentration of the soil in the nature. In alaboratory, the condition “under the salt stress” means a condition thatthe culture fluid for growing a plant contains the sodium chlorideconcentration of which is higher than a sodium chloride concentrationcontained in a normal culture fluid. For example, the condition underthe salt stress is preferably a condition containing 1 wt % to 20 wt %of sodium chloride, or more preferably a condition containing 1 wt % to10 wt % of sodium chloride.

Moreover, in this specification, the description “increasing anexpression level under the salt stress” means an increase in anexpression level of a protein or a gene observed when the protein or thegene is shifted from a normal salt concentration to exposure under theabove-described salt stress. In this case, such an increase alsoincludes the aspect that the protein or the gene does not showexpression at all under the normal salt concentration but showsexpression specifically under the salt stress. Here, the degree of theincrease in the expression level of the protein or the gene is notparticularly limited.

Meanwhile, the nucleic acid of the present invention may be a nucleicacid including part of the nucleic acid sequence of SEQ. ID. NO. 1 inSequence Listing. Here, the description “including part of the nucleicacid sequence of SEQ. ID. NO.1 in Sequence Listing” does notparticularly limit the number of bases therein so far as the nucleicacid includes part of the nucleic acid sequence of SEQ. ID. NO. 1 inSequence Listing.

Furthermore, the nucleic acid of the present invention may becharacterized by including a nucleic acid sequence which hybridizesunder the stringent condition with any one of the nucleic acidsincluding the nucleic acid sequence of SEQ. ID. NO. 1 in SequenceListing, the nucleic acid including part of the above-described nucleicacid sequence, and a nucleic acid having a complementary nucleic acidsequence. In this case, the nucleic acid sequence of the nucleic acid isnot particularly limited so far as the nucleic acid satisfies theabove-described condition. To be more precise, for example, in thenucleic acid, the bases which collectively constitute the nucleic acidsequence of SEQ. ID. NO. 1 in Sequence Listing may include one or morebases of deletion, substitution, insertion, addition or the like. Here,the above-mentioned “deletion, substitution, insertion, and addition”may include not only deletion, substitution, insertion, and additioninvolving a short sequence of 1 to 10 bases, but also include deletion,substitution, insertion, and addition involving a long sequence of 10 to100 bases.

In addition, another nucleic acid of the present invention ischaracterized by including a nucleic acid sequence encoding an aminoacid of SEQ. ID. NO. 2 in Sequence Listing. The nucleic acid sequence isnot particularly limited so far as the nucleic acid encodes for theamino acid.

Next, description will be made regarding a protein of the presentinvention.

The protein of the present invention is characterized by including theamino acid sequence of SEQ. ID. NO. 2 in Sequence Listing. The proteinhas 331 of amino acids. Here, the protein of the present invention maybe a protein, which includes an amino acid sequence having any ofsubstitution, deletion, insertion and addition of one or more aminoacids in the amino acid sequence of SEQ. ID. NO. 2 in Sequence Listing,and which increases an expression level under the salt stress.

Specifically, there are some proteins including amino acid sequenceshaving substitution or deletion of one or more amino acids in the aminoacid sequence of SEQ. ID. NO. 2 in Sequence Listing, and some proteinsincluding amino acid sequences having insertion or addition of one ormore amino acids in the amino acid sequence of SEQ. ID. NO. 2 inSequence Listing, each of which increases a expression level under thesalt stress. All these proteins are deemed to be mutant proteins of theprotein of the present invention of SEQ. ID. NO. 2 in Sequence Listing,and are included in the protein of the present invention so far as theseproteins have the characteristic of increasing an expression level underthe salt stress.

Moreover, sugar chains are added to many proteins and such addition ofsugar chains can be controlled by converting one or more amino acidstherein. In respect of the amino acid sequence of SEQ. ID. NO.2 inSequence Listing, a protein subjected to control of addition of a sugarchain thereto shall be also included in the protein of the presentinvention so far as such a protein has a characteristic of increasing anexpression level under the salt stress.

Next, description will be made regarding a preferred method of isolatinga novel gene according to the present invention.

The novel gene according to the present invention can be isolated by thefollowing Steps (1) to (7), and the isolated gene can be confirmed toexert the salt stress tolerance in the subsequent Step (8).

(1) Isolation of a cDNA Fragment which Increases an Expression LevelUnder the Salt Stress

In order to isolate a cDNA fragment which increases a expression levelunder the salt stress, an adaptable mode is comparing expression levelsof genes between a variety of an object plant considered to haverelatively high tolerance to the salt stress (such as barley K305) and avariety of the object plant considered to have relatively low toleranceto the salt stress (such as barley I743) and thereby isolating the genefound to be increasing the expression level specifically or strongly outof the variety considered to have relatively high tolerance to the saltstress. Moreover, it is also possible to isolate the gene whichincreases the expression level by means of exposing the above-mentionedplants under the salt stress at a higher salt concentration. To be moreprecise, for example, a plant seed may be grown for 12 hours aftergermination by adding 50 to 1000 mM of sodium chloride to a culturefluid, and then roots thereof may be collected and used as a sample.

Next, total RNA and mRNA are prepared out of the sample thus produced.Preparation of the total RNA out of a plant tissue (such as the root, aleaf, or a stem) of the plant targeted for gene isolation may be carriedout by publicly-known methods including the method described in “The PCRexperimental protocol for plants, p. 56, 1999, Shujunsha Co., Ltd.”, forexample. It is also possible to use publicly-known methods of preparingmRNA out of the total RNA thus obtained. Such preparation can be carriedout in accordance with the protocol attached to “Dynabeads Oligo (dT)₂₅”(made by Veritas Corp.), for example.

By using the total RNA or the mRNA thus obtained, isolation of thetargeted gene can be carried out in accordance with a subtractionhybridization method or a differential screening method, for example.The subtraction hybridization method and the differential screeningmethod maybe carried out by use of publicly-known methods. For example,such methods may be carried out in accordance with the protocol attachedto “PCR Select cDNA Subtraction Kit” (made by Clontech).

(2) Northern Hybridization Analysis (Hereinafter Referred to as the“Northern Analysis”)

The Northern analysis using the isolated salt stress tolerantbarley-specific gene as a probe can be performed in order to confirm asto whether the isolated salt stress tolerant barley-specific geneactually increases the expression level or manifests specifically in thesalt stress tolerant barley. The Northern analysis can be carried out bypublicly-known methods, for example, based on the method described in“Experimental protocol without use of isotopes, Vol. 1, DIGHybridization, p. 45, 1994, Shujunsha Co., Ltd.” and the like.

(3) Fabrication of a cDNA Library Specifically Emerging in the SaltStress Tolerant Plant Under the Salt Stress.

A cDNA library can be fabricated by publicly-known methods from the mRNAprepared in Step (1). Such fabrication of the cDNA can be carried out inaccordance with the protocol attached to “Marathon cDNA AmplificationKit” (made by Clontech), for example.

(4) Isolation of a Salt Stress Tolerant Plant-Specific cDNA

Isolation of a salt stress tolerant plant-specific cDNA can be performedby screening the cDNA library, which is fabricated by use of the saltstress tolerant plant as described above, by using a salt stresstolerant plant-specific probe. Such screening can be performed bypublicly-known methods. For example, the method described in theprotocol attached to “AlkPhos Direct system for chemiluminescence” (madeby Amersham Pharmacia Biotech) may be used.

Meanwhile, as for labeling the salt stress tolerant plant-specific cDNAfragment used as the probe, radioisotopes such as ³²P, ³³P or ³⁵S,fluorescent labeling agents, and the like may be used. For example, suchlabeling may be carried out by use of the method described in theprotocol attached to “AlkPhos Direct system for chemiluminescence”.

(5) Base Sequence Determination and Homology Search

The base sequence of the isolated gene can be determined bypublicly-known methods. For example, such determination can be performedin accordance with the protocol attached to “BigDye Terminator CycleSequencing FS Ready Reaction Kit” (made by Perkin Elmer Inc.), forexample. Based on the base sequence determined here, it is possible tocheck for presence and level of homology with any other known genesobtained from other plant species by means of performing homology searchregarding the obtained based sequence by use of a database (such ashttp://www.ncbi.nlm.nih.gov/BLAST/). In this way, it is possible tojudge as to whether the obtained gene is a novel gene or not.

(6) Isolation of an Open Leading Frame of the Salt Stress TolerantPlant-Specific cDNA

In order to isolate only an open leading frame out of theabove-described cDNA, amplification by the PCR method may be performedby use of the cDNA as a template while applying an oligonucleotideprimer containing a start codon (ATG) and an oligonucleotide primercontaining a stop codon. In this event, introduction to theafter-mentioned expression vector is facilitated by performing PCR usingthe oligonucleotide primers, which are arranged by introducing properrecognition sites for restriction enzymes to the 5′ ends of the twooligonucleotide primers mentioned above. Here, the “open leading frame”means the most distant region from the start codon (ATG) to the stopcodon (TGA, TAG or TAA) within the nucleic acid sequence of the cDNA.Isolation of the above-described open leading frame can be performed bythe method described in “The PCR experimental protocol for plants, p.69, 1995, Shujunsha Co., Ltd.”, for example.

(7) Expression of the Open Leading Frame of the Salt Stress TolerantPlant-Specific cDNA

A plasmid is assembled by introducing the open leading frame region ofthe salt stress tolerant plant-specific cDNA isolated in Step (6) to anEscherichia coli expression vector (the pET System such as pET15b).Thereafter, the plasmid is introduced to an Escherichia coli (such as E.coli BL21(DE3)pLysS), and then IPTG is added to a culture fluid for thetransformed Escherichia coli strain. In this way, an induction ofexpression of a protein encoded in the open leading frame becomesfeasible. The above-described induction of expression can be performedby publicly-known methods, such as the method described in “pET SystemManual” (made by Novagen Inc.) or the method described in “The PCRexperimental protocol for plants, p. 9, 1998, Shujunsha Co., Ltd.”.

(8) Measurement of Salt Stress Tolerance of the Transformed Escherichiacoli

As shown in Step (7), the transformed Escherichia coli, in which a geneproduct considered to be related to the salt stress tolerance issubjected to the induction of expression, is cultivated in a culturemedium containing the sodium chloride which concentration is 1 wt % to10 wt %, for example, and a growth rate thereof is measured. In thisway, it is possible to confirm as to whether or not the gene is relatedto the salt stress tolerance.

Next, description will be made regarding fabrication of a transgenicplant made by introducing the gene obtained through the foregoing steps.

The gene obtained through the foregoing steps is considered to have theeffect, which is to provide a plant having the gene with salt stresstolerance by an increase in expression under the salt stress. Therefore,development of a plant having salt stress tolerance is feasible if theabove-described gene can be introduced to a plant which originally doesnot have the salt stress tolerance by use of a genetic engineeringmethod.

As for a method of fabricating such a transgenic plant, the geneobtained in the foregoing steps may be firstly inserted into a cloningvector for a plant cell, and the obtained plasmid may be introduced to aplant targeted for the salt stress tolerance. The cloning vector usableherein includes, for example, binary vector plasmids such as pBI2113,pBI101, pBI121, pGA482, pGAH and pBIG, and intermediate vector plasmidssuch as pLGV23 Neo, pNCAT and pMON200. When a binary vector plasmid isused, the targeted gene may be inserted between boundary sequences (LBand RB) of the binary vector and then introduced into an Escherichiacoli for amplification. Thereafter, the plasmid may be purified andintroduced into a bacterium that belongs to the Agrobacterium genus(such as a strain of Agrobacterium tumefaciens EHA101) for use intransduction of the plant. As for the method of introducing the plasmidto the bacterium, the freeze-thawing method, the electroporation method,and the like are preferably applied.

Transformation of the plant is feasible by infecting the plant with thetransformed Agrobacterium thus obtained. The leaf disk method, theprotoplast method, and the like may be applied as the method oftransformation (Horsch, R. B., Fry, J. E., Hoffmann, N. L., Eichholtz,D., Rogers, S. G., and Fraley, R. T. “A simple and general method fortransferring into plants”, Science 227, 1229–1231 (1985), Kyozuka, J.,Hayashi, Y., and Shimamot, K., “High frequency plant regeneration fromrice protoplasts by novel nurse culture methods”, Mol. Gen. Genet. 206,408–413 (1987)).

Alternatively, it is also possible to apply a method of directlyintroducing the targeted gene to the plant without using theAgrobacterium. To be more precise, such a method includes the particlegun method, the polyethylene glycol method, the liposome method, and themicro-injection method, for example.

Moreover, a host plant for introduction of the gene includes not onlycrops such as barley, rice, corn, tobacco, Arabidopsis, wheat, soybean,and tomato, but also cultured cells, plant organs (for example, root,leaf, petal, rhizome, seed and the like) and plant tissues (for example,epidermis, phloem, parenchyma, xylem, vascular strand and the like)thereof.

Fabrication of a transgenic plant having salt stress tolerance isfeasible by introducing the nucleic acid of the present invention to thehost plant as described above.

EXAMPLES

Now, the present invention will be described in more detail based onexamples. However, it is to be understood that the present inventionshall not be particularly limited to the examples described below.

Example 1

(Preparation of a Root of Salt Stress Tolerant Barley and a Root of SaltStress Sensitive Barley)

A seed of barley K305, which is a variety of salt stress tolerantbarley, and a seed of barley I743, which is a variety of salt stresssensitive barley, were severally subjected to germination. After leavingthe seeds at rest for one day, each of the germs was grown in a potcontaining a 0.25 mM calcium sulfate solution for two days. Then, theseedlings were further grown for one day in a culture solution(containing 4 mM KNO₃, 1 mM NaNO₃, 4 mM NaH₂PO₄, 2 mM CaCl₂, 1 mM MgSO₄,1 ppm Fe, 0.5 ppm B, 0.5 ppm Mn, 0.05 ppm Zn, 0.02 ppm Cu, and 0.01 ppmMo). Thereafter, sodium chloride was added to the culture fluid so as tomake the final concentration 100 mM, and the seedlings were growntherein for additional 12 hours. Then, the roots were harvested andfrozen in liquid nitrogen.

(Preparation of Total RNAs and mRNAs Out of the Root of the Salt StressTolerant Barley and the Root of the Salt Stress Sensitive Barley)

Preparation of total RNAs and mRNAs out of the root of the salt stresstolerant barley K305 and the root of the salt stress sensitive barleyI743 was conducted as follows. Each barley root was frozen and crushedin the liquid nitrogen, and then suspended in a guanidine isothiocyanatesolution (containing 4M guanidine isothiocyanate, 25 mM sodium citrate(pH 7.0), 0.5% sodium N-lauryl sarcosinate, and 0.1 M 2-mercaptoethanol)and centrifuged (10,000 rpm, 15 minutes, 20° C.) therein. Eachsupernatant fluid thus obtained was layered on a cesium chloridesolution and further centrifuged (100,000 rpm, 3 hours, 20° C.) therein.Each of the precipitates thus obtained was dissolved in a TES solution(containing 10 mM Tris-HCL (pH 7.4), 5 mM EDTA, and 1% SDS), andextracted by phenol/chloroform extraction. Thereafter, 1/10 quantity of3M sodium acetate (pH 5.2) and 2.5 quantity of ethanol were addedthereto, and the solution was allowed to stand for one night at −20° C.The solutions which were allowed to stand for one night were centrifugedin 15,000 rpm at 4° C. for 20 min, and then the obtained precipitateswere severally dissolved into water to form total RNA samples. The totalRNA samples were provided to prepare mRNAs thereby in accordance withthe attached protocol with “Dynabeads Oligo (dT)₂₅” (made by VeritasCorp.).

(Isolation of a Salt Stress Tolerant Barley-Specific cDNA Fragment)

An mRNA, which was equivalent to a difference between the mRNA obtainedfrom the root of the salt stress tolerant barley K305 and the mRNAobtained from the root of the salt stress sensitive barley I743, wascollected and used for preparing a salt stress tolerant barley-specificcDNA fragment. Preparation of the salt stress tolerant barley-specificcDNA fragment was conducted as will be described below, while using“PCR-Select cDNA Subtraction Kit” (made by Clontech).

Firstly, cDNAs were synthesized by using the mRNA obtained from the rootof the salt stress tolerant barley K305 and the mRNA obtained from theroot of the salt stress sensitive barley I743, respectively. Next, thesynthesized cDNAs were severally digested with a restriction enzymeRsaI. The obtained cDNA of the salt stress tolerant barley K305 wassplit into two, and adapters severally having different nucleicsequences were ligated to both ends of the cDNAs. The respective cDNAsafter ligation were subjected to hybridization while adding an excessiveamount of the cDNA of the salt stress sensitive barley I743. Then, thesolutions after hybridization were mixed together and again subjected tohybridization while adding the cDNA of the salt stress sensitive barleyI743 which was transformed to a single strand. Thehybridization-completed solution thus obtained was then subjected to PCRwhile using an adapter-specific primer, whereby the cDNA fragment beingpresent only in the root of the salt stress tolerant barley K305 wasamplified. The cDNA fragment, which was the PCR product, was thenutilized for fabrication of a cDNA fragment library while using “PGEM-Tand pGEM-T Easy Vector Systems” (made by Promega Corp.). The cDNAfragment was prepared from this library, and the Northern analysis wasconducted with the cDNA fragment as a probe, which was labeled by using“AlkPhos Direct system for chemiluminescence” (made by AmershamPharmacia Biotech).

The Northern analysis was conducted as follows. Specifically, the totalRNAs of the root of the salt stress tolerant barley and the root of thesalt stress sensitive barley were prepared according to theabove-described method, and the total RNA thus obtained were subjectedto electrophoresis by use of a denatured agarose gel (containing 1.2%agarose, 6.3% formaldehyde, 20 mM MOPS, 5 mM sodium acetate, and 1 mMEDTA (pH 7.0)). The RNAs fractionated within the agarose gel weretranscribed to nylon membranes and then subjected to hybridization whileusing the labeled cDNA as the probe.

Such hybridization was conducted as follows. First, the nylon membraneson which the RNAs have been transcribed were blocked with ahybridization buffer. Thereafter, the probe was added onto thehybridization buffer and the nylon membranes were kept at 55° C. for 16hours. After that, the nylon membranes were subjected to washingprocesses for two times severally for 10 minutes at 55° C. by use of awashing fluid (containing 2 M urea, 0.1% SDS, 50 mM sodium phosphate (pH7.0), 150 mM NaCl, 10 mM MgCl₂, and 0.2% blocking reagent), and further,subjected to washing processes for two times severally for 5 minutes ata room temperature with the washing fluid. After the washing processes,the nylon membranes were dipped in a CDP-Star solution for 5 minutes ata room temperature. Then, detection of bands bonding the cDNA wasperformed. FIG. 1 illustrates results of such detection. In FIG. 1, lane1 shows a result of electrophoresis of the total RNA obtained from theroot of the barley K305 which was not subjected to the salt stresstreatment, lane 2 shows a result of electrophoresis of the total RNAobtained from the root of the barley K305 which was subjected to thesalt stress treatment for 12 hours, lane 3 shows a result ofelectrophoresis of the total RNA obtained from the root of the barleyI743 which was not subjected to the salt stress treatment, and lane 4shows a result of electrophoresis of the total RNA obtained from theroot of the barley I743 which was subjected to the salt stress treatmentfor 12 hours.

As shown in FIG. 1, the mRNA of the salt stress tolerant barley-specificcDNA fragment (the Sub4 gene) did not exist in the roots of the saltstress sensitive barley I743 but emerged strongly and specifically inthe roots of the salt stress tolerant barley K305. Moreover, in the rootof the salt stress tolerant barley K305 being exposed to the salt stressfor 12 hours, an increase in expression of the mRNA of the Sub4 gene wasobserved at about double as compared to the same root before exposure tothe salt stress. Such an aspect indicated that the cDNA fragment of thesalt stress tolerant barley, which was isolated in the above-describedprocesses, emerges and functions specifically in the root of the saltstress tolerant barley K305, and the expression level thereof increasedalong with the salt stress.

Example 2

(Isolation of a Salt Stress Tolerant Barley-Specific cDNA)

A cDNA library was fabricated from the mRNA obtained from the root ofthe salt stress tolerant barley K305 by use of “Marathon cDNAAmplification Kit” (made by Clontech) and “pGEM-T and pGEM-T Easy VectorSystems” (made by Promega Corp.). Colony hybridization was performedwhile using the obtained Sub4 gene fragment as a probe, and then cDNAscreening was carried out.

Such hybridization was conducted under the following condition.Specifically, respective colonies in the cDNA library were transcribedto a nylon membrane, and this nylon membrane was blocked by use of ahybridization buffer. A probe was prepared by labeling the Sub4 genefragment with “AlkPhos Direct system for chemiluminescence” (made byAmersham Pharmacia Biotech), and the nylon membrane finished withblocking was retained at 55° C. for 16 hours together with the probe.Then the nylon membrane was subjected to washing processes for two timesseverally for 10 minutes at 55° C. by use of the above-described washingfluid, and further, subjected to washing processes for two timesseverally for 5 minutes at a room temperature with the washing fluid.Thereafter, the nylon membrane was dipped in a CDP-Star solution for 5minutes at a room temperature. Then, detection of positive coloniesbonding the cDNA was performed to isolate the salt stress tolerantbarley-specific cDNA.

Example 3

(Determination of a Base Sequence of the Salt Stress TolerantBarley-Specific cDNA and an Amino Acid Sequence of a TranslationProduct)

Next, a base sequence of the Sub4 gene being the obtained salt stresstolerant barley-specific cDNA was determined. The base sequence is shownin SEQ. ID. NO. 1 in Sequence Listing. Determination of the basesequence was conducted by use of “BigDye Terminator Cycle Sequencing FSReady Reaction Kit” (made by Perkin Elmer Inc.) and “Genetic AnalyzerABI PRISM 310” (made by Perkin Elmer Inc.).

An amino acid sequence of a translation product estimated from the basesequence of the Sub4 gene determined herein, which is the salt stresstolerant barley-specific cDNA, is shown in SEQ. ID. NO. 2 in SequenceListing. Note that the amino acid sequence of SEQ. ID. NO. 2 in SequenceListing corresponds to the base sequence of SEQ. ID. NO. 1, from the64th start codon (ATG) to the 1057th stop codon (TGA) thereof.

Example 4

(Homology Search)

Homology comparison was carried out by means of comparing the Sub4 genebeing the obtained salt stress tolerant barley-specific cDNA and theamino acid sequence estimated from the base sequence thereof, with knowngenes and amino acid sequences on a data base. As a result, the obtainedgene did not show high homology with other nucleic acids or other aminoacids in terms of the nucleic acid level and the amino acid level. Hencethe obtained gene and amino acid sequence were proved to be a novel geneand a novel protein, respectively.

Example 5

(Isolation of an Open Leading Frame of the Salt Stress TolerantBarley-Specific cDNA)

Isolation of an open leading frame of the Sub4 gene was performed byamplification in accordance with the PCR method while setting the cDNAof the Sub4 gene as a template and using a primer including the 64thstart codon (ATG) as well as a primer including the 1057th stop codon(TGA). The primers used therein were a primer EXN1/Sub4: gcagctgctgctcatatgga acaaaat (SEQ. ID. NO. 3 in Sequence Listing) and a primerEXC1/Sub4: ttgaaggcag gatcctcagg aagtcca (SEQ. ID. NO. 4 in SequenceListing). Note that the primer EXN1/Sub4 is complementary to thesequence of SEQ. ID. NO. 1 from the 49th to 60th positions and from the64th to 75th positions; meanwhile, the 61st G is substituted by C, the62nd T is substituted by A, and the 63rd A is substituted by T,respectively. On the contrary, the primer EXC1/Sub4 is complementary tothe sequence of SEQ. ID. NO. 1 from the 1048th to 1059th positions andfrom the 1063rd to 1074th positions; meanwhile, the 1060th A issubstituted by G, the 1061st A is substituted by G, and the 1062nd G issubstituted by A, respectively.

The PCR reaction was conducted by use of “Advantage 2 PCR Kit” (made byClontech) and in accordance with the protocol attached thereto. Here,conditions of the PCR reaction were defined as: repeating the routine“at 94° C. for 15 seconds, at 55° C. for 15 seconds, and at 68° C. for60 seconds” for 30 cycles; and then at 68° C. for 5 minutes. As aresult, it was able to isolate an open leading frame region of the Sub4gene as long as 1028 base pairs. The open leading frame region of theSub4 gene was sub-cloned to a vector pGEM-T by use of “PGEM-T and pGEM-TEasy Vector Systems” (made by Promega Corp.).

Example 6

(Expression of the Open Leading Frame of the Salt Stress TolerantBarley-Specific cDNA)

A plasmid was fabricated by means of introducing the isolated openleading frame region of the salt stress tolerant barley-specific geneSub4 to an Escherichia coli expression vector while using “pETExpression System plus Competent Cells” (made by Novagen Inc.), and theninduction of expression of the protein was carried out.

First, the above-described plasmid made by introducing the open leadingregion of the Sub4 gene to the pGEM-T was digested with restrictionenzymes NdeI and BamHI. The DNA fragments digested by the enzymes NdeIand BamHI were inserted to recognition sites with NdeI and BamHI inpET15b, in accordance with “pET System Manual” (made by Novagen Inc.).The plasmid pEXsub4 thus obtained was used for transforming E. coliBL21(DE3)pLysS, so as to be subjected to the expression of atranscription product of the open leading frame of the Sub4 gene.Specifically, the transformed E. coli was grown in an LB culture mediumuntil 600-nm absorbance reached 0.4. Thereafter, IPTG (to make 1 mMconcentration) was added to a culture fluid, and the transformed E. coliwas further subjected to shaking culture at 37° C. for 8 hours. Theculture fluid was then centrifuged, and an obtained Escherichia colibacterial cell was suspended in a sample buffer (containing 50 mMTris-HCl (pH 6.8), 4% SDS, and 10% glycerol). After boiling for 10minutes, detection of the protein being manifested the expression wascarried out in a 12% SDS-PAGE. FIG. 2 shows the result. In FIG. 2, lane1 shows an electrophoretic pattern of the Escherichia coli bacterialcell subjected to introduction of only the pET15b, and lane 2 shows anelectrophoretic pattern of the Escherichia coli bacterial cell subjectedto introduction of the pEXsub4.

As shown in FIG. 2, a protein band equivalent to a molecular weight ofapproximately 33,000 was recognized in the Escherichia coli showed theexpression of the open leading frame of the Sub4 gene. Such a molecularweight was extremely close to the molecular weight at 36, 663 of theprotein to be calculated from the amino acid sequence of SEQ. ID. NO. 2,thus the expression of the targeted protein in the transformedEscherichia coli was confirmed.

Example 7

(Salt Stress Tolerance of the Transformed Escherichia coli)

Salt stress tolerance of the transformed Escherichia coli fabricated inExample 6 was examined to ascertain as to whether or not the Sub4 geneis directly related to the salt stress tolerance. The E. coliBL21(DE3)pLysS (pET15b), which was transformed with the plasmid pET15bthat did not contain the Sub4 gene, was used as an object.

The above-described Escherichia coli was cultivated in an LB culturemedium containing ampicillin (to make 50 μg/ml concentration) and thengathered by centrifugation. The gathered Escherichia coli was thensuspended in a new LB culture medium containing ampicillin, andsubjected to shaking culture by 160 rpm at 25° C. for 1 hour.Subsequently, IPTG (to make 0.5 mM concentration) was added to the fluidafter cultivation so as to induce the expression of the introduced gene,and the fluid was again subjected to shaking culture for 1 hour.Absorbance (600 nm) of the fluid was measured, and then the fluid wassuspended in the same culture medium containing IPTG and ampicillinuntil the absorbance (600 nm) reached 0.1. Simultaneously, NaCl wasadded to the LB culture medium so as to make various concentrations of1%, 3%, 5%, and 7%. Those samples were subjected to shaking culture at25° C., and growth rates were measured thereafter. The results are shownin FIG. 3. In the graph, the transformed Escherichia coli containing theSub4 gene was indicated as pSub4-EX, and the Escherichia colitransformed only by the pET15b was indicated as pET15b.

As a result, the E. coli BL21(DE3)pLysS (pEXsub4) transformed by theplasmid containing the Sub4 gene initiated growth irrespective of thevarious NaCl concentrations sooner than the object E. coliBL21(DE3)pLysS (pET15b), and the growth rates were also faster. Suchresults indicated that the salt stress tolerance of the Escherichia coliwas improved by expression of the Sub4 gene in the Escherichia coli.Moreover, the results strongly suggested that the Sub4 gene productoperated as a molecule having a function to improve salt stresstolerance of a plant.

INDUSTRIAL APPLICABILITY

As described above, the nucleic acid and the protein of the presentinvention, which are designed to be subjected to the induction ofexpression under salt stress, are characterized by increasing anexpression level under salt stress. Therefore, the present invention iscapable of providing the novel protein with a function to impart saltstress tolerance to a plant, and providing the novel gene encoding theprotein. Hence, fabrication of a transgenic plant having salt stresstolerance becomes feasible by using the novel gene.

1. An isolated nucleic acid comprising the nucleic sequence of SEQ IDNO:
 1. 2. The isolated nucleic acid of claim 1, which consists of SEQ IDNO:
 1. 3. An isolated nucleic acid comprising a sequence which encodesthe amino acid sequence of SEQ ID NO:
 2. 4. The isolated nucleic acid ofclaim 3, which consists of the sequence which encodes the amino acidsequence of SEQ ID NO: 2.